In chromatography systems, liquids or gases are conveyed through suitable connecting lines between the components of the respective system. Said connecting lines which may be composed for example of high-grade steel have, at their ends, suitable connecting systems, also known as fittings, in order to be able to produce a sealed connection to the ports of the components.
A connecting system of said type was described as early as in the year 1975 in U.S. Pat. No. 3,880,452. Here, a capillary which forms the connecting line between two components is inserted into the capillary receiving opening of a bushing unit or a connecting unit and fixed in the bushing by means of a fastening screw which has a central bore for guiding the capillary. For sealing, one or more sealing elements which surround the capillary in the front end region thereof are pressed into the capillary receiving opening, which runs inward in a conical manner, by means of the fastening screw during the connection of capillary and bushing unit.
It is however a disadvantage here that the sealing action is provided not in the plane of the face surface, which is perpendicular to the longitudinal axis of the capillary, but rather is offset rearward from the face surface by a certain distance in an axial direction. This gives rise to a dead volume which has a disadvantageous effect in particular in the field of high-performance liquid chromatography. To be able to ensure the sealing action of such connections at the extremely high pressures used in high-performance liquid chromatography, use is often made of sealing elements such as are described as prior art for example in FIG. 2 of U.S. Pat. No. 4,619,473. These are sealing elements which are of annular cross section and which are normally likewise composed of high-grade steel and which, in longitudinal section, have a partially conical profile of the outer diameter. Here, the capillary projects with its foremost section through the sealing element into a cylindrical recess (pilot bore) in the bushing element. The bushing element widens conically proceeding from the rear end of the pilot bore, wherein the conical widening has a greater angle with respect to the longitudinal axis of the capillary than the sealing element. As a result, as the sealing element is pressed into the receiving opening by means of a fastening screw, an extremely high radially inwardly directed pressure is exerted on the front region of the sealing element, such that the sealing point is formed here. Said pressure however normally results in a deformation of the sealing element and of the capillary, wherein the sealing element is pressed by way of its front edge annularly into the outer circumference of the capillary.
Such a deformation is undesirable in particular because in this way, the sealing element is connected in a positively locking and non-positively locking manner to the capillary, and the sealing element can no longer be readily displaced in an axial direction on the capillary. If the sealing connection is released and if it is sought to screw a connector element of said type in to another bushing unit, for example because a component of the chromatography system must be replaced, then it is duly possible for a sealed connection to be produced again, but owing to tolerances or manufacturer-dependent differences in the depths of the receiving opening, it can no longer be ensured that the capillary, by way of its face surface, acts again on the face surface of the line to be connected. If the receiving opening of the bushing unit of the exchanged component is longer in an axial direction than in the case of the component used previously, an undesired dead volume is formed. If the receiving opening for the capillary in the exchanged component is shorter in an axial direction than in the case of the component used previously, the capillary is even deformed, possibly damaged, under the pressure of the fastening screw, and a sealed connection is no longer possible under some circumstances. This is because the sealing element that is fastened in a positively locking and non-positively locking manner to the capillary cannot move in an axial direction.
However, in the case of such a fitting, a certain dead volume is also scarcely avoidable if the face surfaces of the capillary and of the line to be connected are situated directly opposite or make contact, because the sealing point is situated not in the region of the face surface of the capillary or of the line to be connected.
To solve said problem, DE 10 2009 022 368 discloses a connector unit in which an annular sealing element is provided on the outer side of the capillaries. Said sealing element is inserted together with the capillary into the pilot bore of a bushing housing and has an axial thrust force exerted thereon such that, as a result of plastic or elastic deformation of the sealing element, sealing of the capillary connection is attained already in the region of the base surface of the pilot bore, whereby the formation of a dead volume is prevented. During the disassembly of the connection, the sealing element can also be pulled out of the pilot bore in a relatively simple manner together with the capillary.
Said connector unit is however suitable only for capillaries which, when inserted into the pilot bore, exhibit an adequate amount of space on the outer side of the capillary for the arrangement of the annular sealing element there. Since the above-described form of the bushing element of U.S. Pat. No. 4,619,473 has become established as a standard and, in the field of HPLC, a pilot bore diameter of approximately 1.6 mm has become established as a standard, the suitability of said plug-in connection is restricted to thin capillaries with outer diameters of considerably less than 1.6 mm, for example 200 μm to 500 μm, such as for example fused silica capillaries or else metal capillaries with an outer diameter of for example approximately 0.8 mm. The connector unit is however unsuitable for capillaries with larger outer diameters, such as for example the capillaries with approximately 1.6 mm outer diameter that are widely used in the field of HPLC.
This substantially also applies to the connector unit known from DE 10 2008 059 897 B4 and to the related connecting system for connecting capillaries, in which the front end region of the capillary, which is preferably composed of metal, is flared in a trumpet-like manner. It is achieved in this way that an annular sealing element which surrounds the front end region of the capillary is pressed with an axial pressing force in the direction of the base of a corresponding receiving opening in a bushing unit or of a connecting unit which has a bushing. Here, too, a sealing action is attained in the region of the front face surface of the trumpet-like end region if the diameter of the end region is smaller than the diameter of the receiving opening. As a result of the exertion of load on the rear face side of the annular sealing element by means of the front region, which is displaceable on the capillary, of the connector housing, the sealing element is pushed forward and compressed such that, as a result of a plastic deformation, a sealing action is attained in the annular space formed by the capillary and by the receiving opening (including the base surface thereof) and by the front side of the connector housing. In this way, not only is a very good sealing action attained, but it is also ensured that, when the connection is released, the sealing element is pulled out of the receiving opening together with the capillary, and cannot remain in the receiving opening.
To solve the problem regarding the possibility of using a bushing unit or connecting unit with a pilot bore for receiving a capillary, which pilot bore has an inner diameter that substantially corresponds to the outer diameter of the capillary, it is proposed in DE 10 2011 050 037 B3 that the sealing element be provided within the wall of the front end of the capillary to be connected. Here, the sealing element forms the front end of the capillary. For stabilization of the capillary wall in the region of the sealing element, use is preferably made of a supporting inner pipe.
Furthermore, capillaries are known which, owing to their metal-free, media-contacting inner surface and high pressure resistance, are of multi-part construction. The inner, media-contacting surface is formed by a plastics hose, generally composed of a polyether ether ketone (PEEK). To attain the pressure stability, the plastics hose is embedded into a metallic pipe. To provide the necessary metal-free connections for this purpose, said capillary ends are insert molded with plastic, such that the metal cannot come into contact with the medium at any location.
Already-known connecting systems used on such capillaries of multi-part construction, that is to say on capillaries whose wall is composed of multiple shell layers, have the disadvantage that, in addition to the problems mentioned above, it is also necessary, in the connecting region, to prevent the medium from coming into contact with the metal shell or with a shell layer composed of non-inert material, and to prevent medium from being able to penetrate between the shell layers if the connection between the shell layers of the capillary does not exhibit adequate stability with respect to the pressure of the medium or if the shell layers are not connected to one another at all.